[dpdk-dev] [PATCH v6 5/7] power: support callbacks for multiple Rx queues

Burakov, Anatoly anatoly.burakov at intel.com
Wed Jul 7 16:35:24 CEST 2021


On 07-Jul-21 1:51 PM, Ananyev, Konstantin wrote:
> 
> 
>> -----Original Message-----
>> From: Burakov, Anatoly <anatoly.burakov at intel.com>
>> Sent: Wednesday, July 7, 2021 12:54 PM
>> To: Ananyev, Konstantin <konstantin.ananyev at intel.com>; dev at dpdk.org; Hunt, David <david.hunt at intel.com>
>> Cc: Loftus, Ciara <ciara.loftus at intel.com>
>> Subject: Re: [PATCH v6 5/7] power: support callbacks for multiple Rx queues
>>
>> On 07-Jul-21 11:11 AM, Ananyev, Konstantin wrote:
>>>>>
>>>>>> Currently, there is a hard limitation on the PMD power management
>>>>>> support that only allows it to support a single queue per lcore. This is
>>>>>> not ideal as most DPDK use cases will poll multiple queues per core.
>>>>>>
>>>>>> The PMD power management mechanism relies on ethdev Rx callbacks, so it
>>>>>> is very difficult to implement such support because callbacks are
>>>>>> effectively stateless and have no visibility into what the other ethdev
>>>>>> devices are doing. This places limitations on what we can do within the
>>>>>> framework of Rx callbacks, but the basics of this implementation are as
>>>>>> follows:
>>>>>>
>>>>>> - Replace per-queue structures with per-lcore ones, so that any device
>>>>>>      polled from the same lcore can share data
>>>>>> - Any queue that is going to be polled from a specific lcore has to be
>>>>>>      added to the list of queues to poll, so that the callback is aware of
>>>>>>      other queues being polled by the same lcore
>>>>>> - Both the empty poll counter and the actual power saving mechanism is
>>>>>>      shared between all queues polled on a particular lcore, and is only
>>>>>>      activated when all queues in the list were polled and were determined
>>>>>>      to have no traffic.
>>>>>> - The limitation on UMWAIT-based polling is not removed because UMWAIT
>>>>>>      is incapable of monitoring more than one address.
>>>>>>
>>>>>> Also, while we're at it, update and improve the docs.
>>>>>>
>>>>>> Signed-off-by: Anatoly Burakov <anatoly.burakov at intel.com>
>>>>>> ---
>>>>>>
>>>>>> Notes:
>>>>>>        v6:
>>>>>>        - Track each individual queue sleep status (Konstantin)
>>>>>>        - Fix segfault (Dave)
>>>>>>
>>>>>>        v5:
>>>>>>        - Remove the "power save queue" API and replace it with mechanism suggested by
>>>>>>          Konstantin
>>>>>>
>>>>>>        v3:
>>>>>>        - Move the list of supported NICs to NIC feature table
>>>>>>
>>>>>>        v2:
>>>>>>        - Use a TAILQ for queues instead of a static array
>>>>>>        - Address feedback from Konstantin
>>>>>>        - Add additional checks for stopped queues
>>>>>>
>>>>
>>>> <snip>
>>>>
>>>>> ....
>>>>>> +static inline void
>>>>>> +queue_reset(struct pmd_core_cfg *cfg, struct queue_list_entry *qcfg)
>>>>>> +{
>>>>>> +     const bool is_ready_to_sleep = qcfg->n_empty_polls > EMPTYPOLL_MAX;
>>>>>> +
>>>>>> +     /* reset empty poll counter for this queue */
>>>>>> +     qcfg->n_empty_polls = 0;
>>>>>> +     /* reset the queue sleep counter as well */
>>>>>> +     qcfg->n_sleeps = 0;
>>>>>> +     /* remove the queue from list of cores ready to sleep */
>>>>>> +     if (is_ready_to_sleep)
>>>>>> +             cfg->n_queues_ready_to_sleep--;
>>>>>> +     /*
>>>>>> +      * no need change the lcore sleep target counter because this lcore will
>>>>>> +      * reach the n_sleeps anyway, and the other cores are already counted so
>>>>>> +      * there's no need to do anything else.
>>>>>> +      */
>>>>>> +}
>>>>>> +
>>>>>> +static inline bool
>>>>>> +queue_can_sleep(struct pmd_core_cfg *cfg, struct queue_list_entry *qcfg)
>>>>>> +{
>>>>>> +     /* this function is called - that means we have an empty poll */
>>>>>> +     qcfg->n_empty_polls++;
>>>>>> +
>>>>>> +     /* if we haven't reached threshold for empty polls, we can't sleep */
>>>>>> +     if (qcfg->n_empty_polls <= EMPTYPOLL_MAX)
>>>>>> +             return false;
>>>>>> +
>>>>>> +     /*
>>>>>> +      * we've reached a point where we are able to sleep, but we still need
>>>>>> +      * to check if this queue has already been marked for sleeping.
>>>>>> +      */
>>>>>> +     if (qcfg->n_sleeps == cfg->sleep_target)
>>>>>> +             return true;
>>>>>> +
>>>>>> +     /* mark this queue as ready for sleep */
>>>>>> +     qcfg->n_sleeps = cfg->sleep_target;
>>>>>> +     cfg->n_queues_ready_to_sleep++;
>>>>>
>>>>> So, assuming there is no incoming traffic, should it be:
>>>>> 1) poll_all_queues(times=EMPTYPOLL_MAX); sleep; poll_all_queues(times=1); sleep; poll_all_queues(times=1); sleep; ...
>>>>> OR
>>>>> 2) poll_all_queues(times=EMPTYPOLL_MAX); sleep; poll_all_queues(times= EMPTYPOLL_MAX); sleep; poll_all_queues(times=
>>>> EMPTYPOLL_MAX); sleep; ...
>>>>> ?
>>>>>
>>>>> My initial thought was 2) but might be the intention is 1)?
>>>>
>>>>
>>>> The intent is 1), not 2). There's no need to wait for more empty polls
>>>> once we pass the threshold - we keep sleeping until there's traffic.
>>>>
>>>
>>> Ok, then:
>>> Acked-by: Konstantin Ananyev <konstantin.ananyev at intel.com>
>>>
>>> Probably worth to put extra explanation here on in the doc,
>>> to help people avoid wrong assumptions😉
>>>
>>
>> I don't see value in going into such details. What would be the point?
>> Like, what difference would this information make to anyone?
> 
> I thought it is obvious: if you put extra explanation into the code,
> then it would be easier for anyone who reads it (reviewers/maintainers/users)
> to understand what it supposed to do.
> 

You're suggesting to put this *in the doc*, which implies that *the 
user* will find this information useful. I'm OK with adding this info as 
a comment somewhere perhaps, but why put it in the doc?

-- 
Thanks,
Anatoly


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